Semiconductor light sources such as light emitting diodes (LEDs) as a liquid crystal backlight or lighting equipment have become prevalent. FIG. 1 is a block diagram of a lighting device including an LED. A lighting device 500r includes an LED light source 502, a rectifying circuit 504, a smoothing condenser 506, and a switching converter 100r. The LED light source 502 is an LED string, and includes a plurality of LEDs connected in series. The rectifying circuit 504 and the smoothing condenser 506 rectify and smooth a commercial alternating current (AC) voltage VAC to convert it into a DC voltage VDC. The switching converter 100r receives the DC voltage VDC as an input voltage VIN and supplies a driving current ILED corresponding to a desired amount of light to the LED light source 502.
The switching converter 100r is a step-down converter, and includes an output circuit 102 and a control circuit 200r. The output circuit 102 includes a smoothing capacitor C1, a rectifying diode D1, a switching transistor M1, an inductor L1, a detection resistor RCS, a capacitor C11, and resistors R11 and R12.
During an ON period of the switching transistor M1, a coil current IL1 flows to the detection resistor RCS by way of the inductor L1 and the switching transistor M1. A voltage drop (current detection signal) VCS of the detection resistor RCS is fed back to a current detection (CS) terminal of the control circuit 200r. The control circuit 200r generates a pulse signal whose duty ratio is adjusted such that a peak of the current detection signal VCS becomes close to a target voltage corresponding to a target amount of light of the LED light source 502, and switches the switching transistor M1 connected to an OUT terminal. A driving current ILED smoothed from the coil current IL1 is supplied to the LED light source 502.
A voltage VZT that is based on a drain voltage VD of the switching transistor M1 is input to a zero cross detection (ZT) terminal of the control circuit 200r through the capacitor C11 and the resistors R11 and R12. The control circuit 200r detects that the coil current IL flowing through the inductor L1 becomes zero (zero cross) according to the voltage VZT, and turns on the switching transistor M1. Further, the control circuit 200r detects that the current detection signal VCS reaches a target voltage VREF, and turns off the switching transistor M1.
A DC voltage VDC is input to a high voltage (VH) terminal of the control circuit 200r, and a capacitor C21 is connected to a power source (VCC) terminal. The control circuit 200r includes a start-up circuit 202 installed between the VH terminal and the VCC terminal. Since the DC voltage VDC is a high voltage of 100V or higher, the start-up circuit 202 is configured by using a high withstand voltage device.
When a user of the lighting device 500r turns on a switch for lighting on and off the lighting device 500r, the AC voltage VAC is supplied to the rectifying circuit 504 and the voltage VDC of the smoothing condenser 506 is increased. In response to turning on the switch, the control circuit 200r is started up. When the control circuit is started up, a start-up current flows from the VH terminal to the capacitor C21 by way of the start-up circuit 202 and the VCC terminal so that the capacitor C21 is charged.
A voltage VCC of the capacitor C21 is a power source voltage of the control circuit 200r. The control circuit 200r is operable when the power source voltage VCC exceeds a predetermined threshold voltage (lowest operating voltage) VUVLO, and starts to switch the switching transistor M1. Thus, the LED light source 502 is lighted.
The present inventors reviewed the switching converter 100r of FIG. 1 and recognized the following technical problem.
A user may want to set up a time (start-up time) for the LED light source 502 to actually be lighted at a desirable time after the user turns on the power. In a related-art, a fluorescent lamp or an incandescent lamp is lighted through a start-up time which is relatively long until it is lighted after a switch is turned on, but a start-up time of a semiconductor light source such as the LED light source 502 is shorter than that of the fluorescent lamp or the incandescent lamp, and the use of the LED light source 502 in combination with the fluorescent lamp or the incandescent lamp causes disharmony. Further, even when the LED light source 502 is used alone, it may be desirable that the start-up time of the LED light source 502 is relatively long.
Here, in order to clarify the technical problem, the lighting device 500r having the LED light source 502 has been described as an example, but a start-up time taken from a time at which power is applied to a time at which a load is operated may be set, even for the switching converter 100r for supplying power to a certain load in applications other than the lighting device 500r. 